Myelin basic protein (BP) presents a unique opportunity for a detailed study of the relationship between structure and biological activity--BPs from several species have been completely sequenced and there is an impressive fund of information on its encephalitogenic activity in several species, its ability to induce and bind antibody and its ability to bind other biochemical compounds or structures, specifically membrane constituents. In order to study the relationship between structure and activity, we have prepared several well-characterized fragments of the protein by controlled proteolytic digestion. Our purpose is to test the ability of these peptides to replicate the biological activity of the protein. By using fragments from different regions of the protein one can "map" active sites and define the minimal structural requirements for activity.